Irradiation of food products has developed as an industry over a period of approximately 30 years. Use of irradiation is driven by increasing incidence of sickness and death caused by food-borne pathogens. At the present time, FDA regulations have been promulgated for irradiating wheat, wheat powder, potatoes, seasonings, pork, poultry, red meat (e.g. beef and veal), eggs, and produce. These regulations provide for giving absorbed radiation doses to food (other than spices) of up to 100 kilorads. It is expected that eventually many additional foods will be cleared for irradiation for preservation and other beneficial purposes, in addition to disinfestation purposes. A wide spectrum of food products will be covered under such regulations, including ready to eat foods.
At the present time, irradiation of food products is the only commercially viable technology sufficiently effective at destroying harmful microbes or insects on or in a raw or ready to eat product. Nonetheless, consumer sensitivity to the idea of radiation and reaction to the inadequate quality of irradiated food products has slowed commercialization of irradiation. For example, irradiation of meats typically has an immediate negative impact on palatability, depending on the dose used. Meats or meat products that have been irradiated often acquire a characteristic odor and flavor which has been described as irradiation burned, wet dog, or metallic. The higher the dose, the greater the generation of off-flavors and aromas. Irradiation at high doses required for effective reduction of microbes or insects may make the food product unpalatable.
Several methods for reducing objectionable off odors and flavors associated with irradiated meats have been developed. These methods include freezing the meat and irradiating it at very low temperatures; irradiating in the absence of oxygen under vacuum or in the presence of an inert atmosphere; storing the meat products at room temperature after irradiation; and adding an antioxidant, a nitrite, a preservative, or certain other chemical agents, such as mannitol, sodium fumarate, or monosodium glutamate.
Irradiation of food products typically employs one of three types of ionizing radiation: 1) gamma rays from radioisotopes, 2) X-rays generated by energetic electron bombardment on hard metal targets, or 3) direct bombardment with energetic electrons. Gamma and x-ray radiation exhibit similar frequencies and energy; both are electromagnetic waves and physically the same. In fact, low energy gamma rays and X-rays of the same energy differ only in the manner in which the radiation is generated. The former is generated by nuclear processes within a radioactive nucleus, while the later arises from acceleration of energetic electrons by electric (Coulomb) forces from atomic targets.
It is common in the design of irradiators to utilize radioisotopic sources, e.g., Cobalt-60 and Cesium-137. Recently, Cesium-137 sources have been made available through the Department of Energy; and these sources are generally in the form of WESF capsules containing 40-50 kilocuries. A typical apparatus for irradiating a food product places the food product automatically into a thick walled, shielded chamber also housing rods of the radioisotope. Racks of rods provide proper orientation of the isotope for product irradiation. The total dose of gamma radiation received by the food products is determined by exposure time, location of the product within the chamber, and the linear attenuation coefficient of the absorber, which in this case is the food product receiving the radiation. As the emission of gamma-rays from radioactive materials cannot be turned off, the isotopes are submerged in a deep pool of water for safe storage when the irradiator is not in use.
X-rays are produced by high voltages from electrostatic or inductive generators, which accelerate electrons to extremely high energies. After acceleration, the electrons are directed onto a target of a metal having a high atomic number, e.g., tungsten, to produce bremsstrahlung x-rays. There are several types of electron accelerators, such as Van der Graff, betatrons, synchrotrons, and linacs, that are useful for food irradiation. The impact of energetic electrons produces x-rays through two atomic collision processes. First, after collisions, decelerating energetic electrons emit bremsstrahlung. Second, outer bound electrons of the atom replace inner-shell electrons that have been knocked out by incident energetic electrons thus emitting characteristic x-rays. Bremsstrahlung x-rays exhibit energy directly proportional to the energy of incident electrons. Also, as the electron current incident on the target increases, the intensity of x-ray emission will increase proportionally.
Electrically powered x-ray devices advantageously do not employ radioactive materials. Furthermore, X-ray machines can be turned off since they are driven electrically, so they do not require storage in deep pools of water when not in use. This makes X-rays easier to use than radioactivity for irradiation of food products.
Nonetheless, whether conducted with X-rays, radioisotopes, or direct electron beams, irradiation of food products can have the same detrimental effects on consumer acceptance and flavor of the food. The present method and system effectively reduces the microbial burden in or on a food product, while providing a palatable irradiated food product.